This invention relates to overbased metal sulfonates, and more particularly, to an improved process for preparing overbased alkaline earth metal sulfonates in which the formation of crystalline metal carbonates is eliminated.
Calcium sulfonate additives are generally produced by carbonating a hydrocarbon solution of a "neutral" calcium sulfonate, a calcium base (usually calcium hydroxide and/or calcium oxide), and various reaction promoters which include low molecular weight alcohols (usually methanol), water, ammonium carboxylates (usually ammonium formate), and calcium chloride. During the reaction the calcium base reacts with gaseous carbon dioxide to form calcium carbonate. The calcium carbonate is incorporated into small inverted micellar structures during this process; these basic calcium sulfonate micelles effectively render the calcium base oil soluble, a necessary step for this product to be useful in a lubricating oil. The final product must be filtered or centrifuged to remove unreacted solids, particularly since an excess of the calcium base is used in the production of highly basic sulfonates [in the 300-500 Total Base Number (TBN) range]. In this process, the amount of gaseous carbon dioxide charged to the system is critical in order to prevent a phenomenon termed "overcarbonation". If the CO.sub.2 charge exceeds 80-95% of that theoretically required to react with the calcium base, the calcium carbonate incorporated into the product begins to crystallize, resulting in unacceptable products. Generally an "overcarbonated" product will filter extremely slowly and will have a low TBN value. In addition, "overcarbonated" products are often unacceptably hazy, due to the presence of large micellar structures containing crystalline calcium carbonate. Although the onset of complete crystallization is usually very rapid, small amounts of crystalline carbonate may form earlier in the reaction, presumable due to small inhomogeneities in large scale reactor mixing. Partially crystalline products are also undesirable, even if they are not hazy, since they are known to have poor stability characteristics in blends with other common lubricant additives.
Thus, an object of the present invention is to provide an improved process to prepare overbased alkaline earth and other metal detergents having superior stability and resistance to the formation of crystalline calcium carbonate; this process also results in a reduction in solids waste and improved feedstock utilization.